1. Field of the Disclosure
The present disclosure concerns apparatus and methods to accurately compute vertical position.
2. Background of the Disclosure
Modern construction and agricultural tasks often require a high degree of vertical accuracy. In fact, contracts for such projects, such as building a new highway or airport runway often have bonus fee schedules based on the extent to which the finished roadway is smooth and flat. Further, many projects include strict timetables that may also incorporate incentives for on time or early completion, and/or penalties for delays. Hence, it has become very important for contractors to complete construction projects such as roadways or runways, for example, quickly and with a high degree of vertical accuracy, often 1 centimeter, 2-sigma accuracy.
Heavy equipment such as pavers have traditionally used a variety of techniques to create smooth, flat surfaces with a high degree of precision. One low tech method commonly employed to construct smooth, flat roadways is to position hubs at periodic intervals along either side of the road site. Pins are then inserted into the hubs, and a stringline is tied to the pins and pulled taught at the appropriate height for the roadway under construction. The paver's operator then manually attempts to follow the stringline so the resulting asphalt layer is deposited at the appropriate place and depth.
Pavers consist of two main components, the tractor and the screed. The tractor receives, mixes and spreads the asphalt onto the surface to be paved, while propelling the paver forward. The screed compacts and smoothes the deposited asphalt to the desired width and thickness. Therefore, accurately controlling the vertical position of the screed is critical to achieving a smooth, level roadway.
Pavers employ two sensing wands for control purposes: one wand to control elevation; the other to control left/right positioning. The elevation-sensing wand skims the underside of the stringline (or guideline) and is used to control the elevation of the paver's screed (that portion of the paver that compacts and smoothes the asphalt). The alignment-sensing wand skims along the inside of the stringline and is used to control the left/right position of the paver.
However, stringline systems have numerous drawbacks. For example, setting up and taking down a stringline is very time consuming, usually requiring a separate crew. Further, setting up and taking down stringlines is expensive (estimated costs for stringlines range from $10,000 to $16,000 per mile). More importantly, stringlines, particularly those fabricated from polyethylene rope, are prone to sag. To combat sag, aircraft cable has been employed in place of polyethylene rope, however, stretching aircraft cable to avoid sag requires a winch, which in turn consumes additional time and equipment to set up.
In the past several years, two alternatives to the use of stringlines have emerged: Global Navigation Satellite Systems (“GNSS”) and Universal Total Stations. For this application, GNSS refers to any satellite navigation system, including but limited to GPS, GLONASS, GALILEO and COMPASS.
GNSS employs an antenna and receiver mounted on the paver to control the elevation and left/right position of the machine. In addition, a second receiver can be employed as a base station to provide Real Time Kinematic (“RTK”) or a carrier phase enhanced GNSS, with its attendant higher accuracy. RTK or carrier phase enhanced GNSS are well known in the art of global satellite navigation systems.
However, there are several drawbacks to using GNSS for this application. For example, GNSS systems are substantially less accurate for measuring vertical position compared to horizontal position, based on (among other things) the position of the GNSS satellites in view at a given time, the unevenness of the earth's surface compared to the ellipsoid approximation of the earth's surface (employed by most GNSS receivers), along with propagation delays and multi-path errors introduced by the earth's atmosphere. Finally, GNSS systems suffer from latency issues, for example, latency in the reference receiver data due to telemetry delay. Hence, GNSS systems alone may not be sufficient for activities that require a great degree of vertical accuracy, in other words, applications that require vertical accuracy of one centimeter or less.
Another alternative to the use of stringlines is a Universal (or Robotic) Total Station (hereafter “UTS”), which uses a combination of laser and radio telemetry technologies to track a moving prism. For example, UTS systems usually employ a pair of prisms mounted on the left and right side of a paver's screed. Two total stations are positioned along each side of the roadway, each consisting of a tripod-mounted laser and a radio modem. The laser beam of each total station is set at a desired elevation and is detected and reflected by the prisms mounted on the either end of the screed. A change in elevation of the screed's prisms as the screed moves relative to the known elevation of the laser beam is then detected. Onboard signal processing applications assist the operator to follow the location and elevation of the two laser beams as detected by the prisms mounted on the screed.
UTS systems have known drawbacks. For example, UTS systems must be constantly moved, re-installed and re-leveled approximately every 100 meters or 200 meters as the paver moves forward. Even though two sets of UTSs can be leapfrogged (i.e., one system is in use while another is moved up the roadway and set up), this leapfrogging requires additional equipment and manpower. Further, UTS systems are susceptible to environmental interference such as rain, snow, dust and wind.
Therefore, there is a need for an improved control system for construction, agricultural and other heavy equipment that is capable of 1 centimeter vertical accuracy, that does not require stringlines, and avoids the above-mentioned drawbacks of GNSS and UTS systems.